Pathogenic Nematodes: Life Cycles and Clinical Signs

Trichuris trichiura

• Life cycle: Ingest embryonated egg → intestine → adult → eggs in feces → soil → ingestion.
• Infective stage: Embryonated egg.
• Location: Ileocecal and large intestine.
• Vector: None.
• Causes: Dysentery, anemia, and rectal prolapse.
• Identification: Barrel-shaped egg with 2 polar plugs.

Ascaris lumbricoides

• Life cycle: Ingest egg → duodenum → blood → lungs → pharynx → swallowed → duodenum → adult.
• Infective stage: Embryonated egg.
• Location: Duodenum and small intestine; larvae migrate through lungs.
• Vector: None.
• Causes: Pneumonitis, malnutrition, and intestinal blockage.
• Identification: Bumpy/mammillated egg and 3 lips.

Necator americanus

• Life cycle: Egg → J1 → J3 → skin → blood → lungs → pharynx → swallowed → small intestine.
• Infective stage: J3 filariform larva.
• Location: Small intestine; larvae in skin and lungs.
• Vector: None.
• Causes: Anemia and protein malnourishment.
• Identification: Hooked adult, cutting plates, and thin-shelled egg.

Strongyloides stercoralis

• Life cycle: J1 → soil → J3 → skin → blood → lungs → swallowed → small intestine → J1.
• Infective stage: J3 filariform larva.
• Diagnostic stage: J1 in stool.
• Location: Small intestine; larvae in skin and lungs.
• Vector: None.
• Causes: GI symptoms, skin rash, and autoinfection/hyperinfection.
• Identification: Larva in stool with esophageal bulb and genital primordium.

Wuchereria bancrofti

• Life cycle: Mosquito → person → lymphatic vessels → microfilariae → mosquito.
• Infective stage: L3 larva.
• Location: Lymphatic vessels.
• Vector: Mosquito.
• Causes: Elephantiasis and massive swelling.
• Identification: Microfilariae in blood.

Onchocerca volvulus

• Life cycle: Blackfly → person → subcutaneous nodules → microfilariae → skin/eyes → blackfly.
• Infective stage: L3 larva.
• Location: Subcutaneous nodules, skin, and eyes.
• Vector: Blackfly.
• Causes: River blindness and dermatitis.
• Identification: Microfilariae.

Loa loa

• Life cycle: Deer fly → person → tissues → microfilariae in blood → deer fly.
• Infective stage: L3 larva.
• Location: Subcutaneous tissues and eye.
• Vector: Deer fly.
• Causes: Eyeworm, eye irritation, and swelling.
• Identification: Worm can cross the eye.

Dirofilaria immitis

• Life cycle: Mosquito → dog/cat → heart/pulmonary vessels → microfilariae → mosquito.
• Infective stage: L3 larva.
• Location: Heart and pulmonary vessels in dogs/cats; lung lesions in humans.
• Vector: Mosquito.
• Causes: Heartworm and human lung granuloma.
• Identification: Threadlike adult or microfilaria.

Dracunculus medinensis

• Life cycle: Infected copepod swallowed → larva → adult female → skin → worm emerges → larvae enter water → copepod.
• Infective stage: L3 in copepod.
• Location: Subcutaneous tissue and skin.
• Vector: Copepod (water flea).
• Causes: Painful blister and worm emergence.
• Identification: Long thin worm emerging from the skin.